“Super Jelly”: It won’t break no matter how you crush it, the Nokia of jelly!

When we were young, the colorful and variously shaped jellies on the supermarket shelves always attracted our attention. It seemed as if they would fly into our mouths in the next second, bouncy and breaking into pieces as soon as we bit them.

In our impression, jelly is soft and "fragile".

Now, scientists have invented a magical "jelly" that can remain intact even if crushed by a car. It is truly the "Nokia" among "jelly".

Figure | The material remains intact despite repeated crushing by a car (Source: The paper)

Recently, a team led by Professor Oren Scherman from the Melville Polymer Synthesis Laboratory at the University of Cambridge in the UK prepared a high-performance glassy gel-like supramolecular polymer network (SPNs).

This polymer still has ultra-high compressive strength, cyclic stability and rapid room temperature self-recovery at a water content of up to 80%, giving it great prospects in applications such as soft robotics, tissue engineering and wearable bioelectronics.

(Source: Nature Materials)

The related research paper, titled “Highly compressible glass-like supramolecular polymer networks”, has been published in the scientific journal Nature Materials.

Jelly, a hydrogel

Jelly is actually a hydrogel, and gel refers to a sol that loses its fluidity and becomes a liquid-rich semi-solid substance.

Gel materials can actually be found everywhere in our lives. In addition to jelly, grass jelly and the pearls in pearl milk tea are all hydrogels; the contact lenses we wear and the fever-reducing patches we use when we have a fever are all made of gel materials.

Supramolecular polymer gel is also a type of gel. It is a very important soft material. This material is assembled based on multiple non-covalent interactions in three main ways:

1) Supramolecular polymer gels based on hydrogen bonding;
2) Supramolecular polymer gels based on metal coordination;
3) Supramolecular polymer gels based on host-guest molecular recognition.

The reversibility of this non-covalent interaction mode gives supramolecular polymers many properties different from ordinary polymers, such as stimulus responsiveness under light, electricity, heat and other conditions.

This property makes it of great significance in the development of smart materials. Smart wearables, drug delivery, biomaterials, etc. are inseparable from supramolecular polymer materials.

It is well known that the way materials behave, whether they are soft or hard, brittle or strong, depends on their molecular structure.

Supramolecular polymer hydrogel materials have many interesting properties. In addition to stimulus response, they also have strong toughness and self-healing ability. However, it is a big challenge to manufacture hydrogel materials that can withstand high pressure without being crushed.

How is “Super Jelly” made?

The team used barrel-shaped molecules called cucurbiturils to create the hydrogel, which can withstand pressure.

Cucurbituril, also known as cucurbitacin, is a cross-linked molecule with high rigidity and special structure. It is shaped like a gourd with open cavities at both ends. Its cavity can accommodate two guest molecules, like molecular "handcuffs".

The researchers designed guest molecules to stay inside the cavity longer than normal, which allows the polymer network to remain tightly connected, allowing it to withstand stronger pressures.

Figure | Design of glassy SPNs. a. Schematic diagram of the dynamic mechanism of SPNs (above); b. Molecular structures of perfluorophenyl (5FBVI), substituted phenyl (RBVI) ​​guests and host macrocycles; c. Ternary complexation equilibrium including the first and second associated hosts to enhance polar π-π interactions and their related kinetic parameters; d. The supramolecular polyacrylamide network cross-linked by the above interactions and two RBVI guests exhibits slow dissociation kinetics and high compressive strength. (Source: This paper)

80% of this polymer is water, which is about 10% higher than the water content of the longan fruit we eat. Longan will break when you put it in your mouth and bite it (the bite force of an adult is about 50 kg), while the polymer in this study can withstand the weight of a car (1200 kg).

It is conceivable how much pressure-bearing capacity and good compression performance this polymer has, and such a large pressure-bearing capacity is very difficult for hydrogel materials; at the same time, it is incredible to combine good elasticity and strong compressive resistance in the same material.

Not only that, the material has good cyclic compressibility and will not deform or break even after being repeatedly crushed by a car.

Regarding the mechanical properties of the material, Dr. Zehuan Huang, the first author of the article, said, "To make a material with the mechanical properties we wanted, we used a slow-dissociating, non-covalent cross-linker to connect the two molecules together through a chemical bond."

What’s even more amazing is that the pressure-bearing capacity of “super jelly” can be adjusted within a certain range. Dr. Jade McCune, co-author of the article, said, “We found that by simply changing the chemical structure of the guest molecules in the handcuffs, we can easily control the compressive strength of the material.”

This is just like when we make dumplings during Chinese New Year. If the dumplings are filled with normal stuffing, they will taste soft and fragrant. But if you put coins in the dumplings, they will definitely break your teeth.

In the same way, in addition to the adjustable compressive strength, the state of this material is also adjustable. The team selected specific guest molecules for the "handcuffs". Changing the molecular structure of the guest molecules can greatly "slow down" the dynamics of the material, and ultimately the mechanical properties of the material change from rubber to glass.

Therefore, this gel-like SPNs material with extremely high compressibility and ultra-high strength is an important milestone in high-performance soft materials.

In this regard, Dr. Zehuan Huang also said, "As far as we know, this is the first time this material has been made. We are not just writing something new in textbooks, we are opening a new chapter in the field of high-performance soft materials."

"Jelly" becomes shoe sole

With the continuous development of science and technology, high-performance pressure sensors are of great significance in research fields such as electronic skin, wearable devices and soft robots.

However, to date, the sensing range of most hydrogel-based pressure sensors is relatively narrow, limited to lower pressures below 400 kPa, which restricts their application in high-pressure sensing.

To highlight the material's application in bioelectronics, the researchers fabricated a hydrogel-based capacitive pressure sensor and made the surface of the SPNs into a hemispherical structure to improve the material's sensitivity.

Research shows that this hemispherical sensor can withstand ultra-high working pressures of up to 2.5 MPa (1 MPa=1000 kPa), while showing a sensitivity 3 to 4 times higher than that of planar pressure sensors.

The researchers placed the sensor on the soles of the feet and successfully achieved real-time monitoring of three movements (walking, jumping, and standing).

During real-time monitoring, the human body walked, jumped, and stood for 10 seconds. In each action, the researchers observed consistent changes in capacitance, and the material was able to fully recover after each deformation.

Combined, these data suggest that the material's ultra-high compressive properties make it potentially applicable in prosthetics, sensory touch in arms, robotic skin, and more.

Figure | Human body compression movement monitored by capacitive pressure sensor based on SPNs. a. Demonstration diagram of dome structure pressure sensor installed on 80 kg human foot for sensing human movement; b. Real-time monitoring diagram of capacitance variation over time for three human movements: walking, jumping, and standing; e. Schematic diagram of capacitive pressure sensor with hemispherical surface structure; f. Relative capacitance and pressure diagram showing sensitivity.

Perhaps in the near future, this material can bring about tremendous changes in fields such as artificial muscles, tissue engineering, soft robotics and wearable bioelectronics.

However, can this gel-like supramolecular polymer compete with traditional covalent polymers (including proteins, silk, starch, rubber, etc. that are common in our lives)?

Do you think this material can replace the plastic in our lives in the future?

Welcome to leave a message in the comment area~

References:
https://www.nature.com/articles/s41563-021-01124-x
https://www.eurekalert.org/news-releases/935677

https://pubs.acs.org/doi/abs/10.1021/acs.chemrev.5b00369

Written by: Hao Jing Edited by: Kou Jianchao Layout by: Li Xuewei

Source: Academic Headlines